Keep in mind it will be easier to hire PHP dev's than ruby developers. If you find a good ruby dev you will have to pay $$$$.
If Ruby devs are paid more, it's probably because they're more productive. Now, I'm not a Ruby fanboy, and people can be very productive in many other languages. I think in general paying someone far less to write a codebase in PHP may be penny-wise but pound-foolish.
PHP is, in my opinion and that of many others, a very poor language that encourages poor coding practices. It's possible to write good code in PHP and poor code using Django/Rails but I think there's a strong correlation between the tools used and results produced.